A semiconductor integrated circuit (IC) is generally susceptible to an electrostatic discharge (ESD) event, which may damage or destroy the IC. An ESD event refers to a phenomenon of electrical discharge of a current (positive or negative) for a short duration during which a large amount of current is provided to the IC. The high current may be built-up from a variety of sources, such as the human body. To protect an IC from an ESD event, traditional NMOS and PMOS devices are used to discharge an ESD current. However, traditional NMOS and PMOS devices may not respond quickly to an ESD event, and may require additional devices to facilitate triggering. Examples of known ESD protection schemes using additional devices to quickly conduct an ESD current are shown in FIGS. 1 and 2.
FIG. 1 is a reproduction of FIG. 2 of U.S. Pat. No. 5,631,793 (the '793 patent) to Ker, one of the inventors of the present invention. The '793 patent is entitled “Capacitor-Couple Electrostatic Discharge Protection Circuit” and describes an ESD protection circuit using a gate-couple technique in order to fast conduct an ESD current during an ESD event. To achieve the capacitor-couple effect, a resistor-capacitor circuit is coupled to the gates of MOS devices such that a portion of an ESD voltage can be coupled to the gates of the MOS devices, thereby raising the gate voltages and therefore lowering the snapback breakdown voltage of the device.
FIG. 2 is a reproduction of FIG. 3 of U.S. Pat. No. 6,147,538 (the '538 patent) to Andresen, entitled “CMOS Triggered NMOS ESD Protection Circuit”. Andresen describes an ESD protection circuit using a substrate pumping technique. Pumping of a substrate, i.e., increasing substrate potential or applying a small positive bias voltage to the substrate, aids turn on of a parasitic transistor. Additional devices such as resistors and capacitors are also used in the '538 patent to achieve fast conduction of an ESD current.
A limitation with the '793 patent and the '538 patent is that the additional devices, e.g., capacitors and resistors, are passive. Such passive devices require large area for fabrication and complicate making protection circuits.